The present application is the national stage under 35 U.S.C. 371 of PCT/SE98/01807, filed Oct. 7, 1998.
The present invention relates to a mast base.
More particularly, the invention relates to a mast base of the kind comprising a rigid body, intended to support a mast mounted thereon, and four supporting legs serving to support said body in a position in which it is elevated from a substrate for the mast base, said supporting legs extending laterally from the body in mutually diverging directions and being provided, at their outer ends, with supporting feet, by which they may rest on said substrate, each supporting leg comprising an upper arm which, at its inner end, is pivotally connected to an upper portion of the body for pivoting movement relatively to the body around a first horizontal pivot axis, and a lower arm, which, at its inner end, is pivotally connected to a lower portion of the body for pivoting movement relatively to the body around a second horizontal pivot axis, said two arms of each supporting leg being pivotally connected to each other at their outer ends for pivoting movement relatively to each other around a third horizontal axis and at least one of said two arms having a variable length.
Especially, if the mast base is intended to support an antenna mast having directional antennas affixed thereto, it is of outmost importance in a mast base of the above kind that it is possible to counteract any tendency to a variable rotation of the body of the mast base and the mast supported upon said body around a vertical axis that may be caused by wind load on the mast and the antennas affixed thereto. For this reason, in previously known mast bases of said kind, for instance in tha mast base disclosed in WO 93/17208, a plurality of stay wires have been mounted between the supporting feet of the supporting legs as well as between said feet and the lower portions of the body of the mast base. However, such an arrangement is unfavourable in several respects. Firstly, it implies that there do not occur any elements, sticking up from the substrate, that may obstruct a location of the various stay wires in their desired positions. Moreover, such an arrangement cannot be utilized at all to effectively prevent a rotation of the body of the mast base when the horizontal dimensions of said body are small in comparison with the length of the supporting legs.
The invention therefore has for its purpose to provide an improved mast base of the kind initially specified, which offers a favourable solution of the problem above described.
The mast base, according to the invention proposed for said purpose, is primarily characterized in that the upper arm of each one of two supporting legs, which are located at mutually opposite sides of the body and which extend laterally from the body in mutually opposite directions, is pivotally connected to the body by means of two pivot joints, which are located spaced apart in the direction of the first horizontal pivot axis.
The invention eliminates the previous need for a plurality of stay wires, located at the lower end of the mast base, and makes it possible to hold the body firmly against any rotation thereof even in such cases, when the horizontal dimensions of the body are small in comparison with the length of the supporting legs.
According to a preferred embodiment of the invention, said two pivot joints between the body and the upper arm of each one of said two supporting legs may suitably be located at or near mutually different ones of two diagonally opposite corners of the body which, as seen in plan view, may have a generally square cross-sectional shape.
Furthermore, the upper arm of each one of said two supporting legs may preferably comprise two branches which extend in mutually converging directions towards the outer end of said arm from respective ones of said two pivot joints. Such a construction of said arm is especially favourable as a consequence of the fact that it will result in that the stiffness against lateral flexure of said arm will become very high.
In order to facilitate a location of the mast base upon a flat roof of a house or any other substrate, where various obstacles formed by elements sticking up from the substrate occur, at least one of the two other supporting legs, which are located at mutually opposite sides of the body of the mast base and the two firstmentioned supporting legs, may suitably be mounted for limited pivotal movement in a lateral direction around an at least approximately vertical pivot axis, located at the inner end of said leg.
Moreover, at its lower end, the body of the mast base may be provided with means serving to support a ballast. Such a design of the body eliminates the need for securing the feet of the supporting legs to the substrate or loading them with weights. As a consequence, the lower arms of the supporting legs will be subjected to tensile forces only, which means that said arms may be dimensioned substantially weaker than would be required if said arms are subjected also to compressive forces.